The Quantum Structure of Space and Time (293 pages)

Quantum Mechanics 25

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3

c

x

14000,. ,. ,. ,. , , 1

4000 spectrometer background level

58 59 60 61 62 63

position of 3rd grating (pm)

Fig. 2.2 Interference of Biomolecules. The molecule tetraphenylporphyrin (C44H30N4) is shown

at left. Its quantum interference fringes in a Talbot-Lau interferometer are shown at right from

experiment carried out in Anton Zeilinger’s group (Hackermiiller, et a1 (2002)).

ingenuity and sophistication have extended the regime in which quantum mechanics

has been tested. No limit to its validity has yet emerged.

Even while acknowledging its undoubted empirical success, many of our greatest

minds have rejected quantum mechanics as a framework for fundamental theory.

Among the pioneers, the names of Einstein, Schrodinger, DeBroglie, and Bohm

stand out in this regard. Among our distinguished contemporaries, Adler, Leggett,

Penrose, and ’t Hooft could probably be counted in this category. Much of this

thought has in common the intuition that quantum mechanics is an effective ap-

proximation of a more fundamental theory built on a notion of reality closer to that

classical physics.

Remarkably, despite eighty years of unease with its basic premises, and despite

having been tested only in a limited, largely microscopic, domain, no fully satisfac-

tory alternative to quantum theory has emerged. By fully satisfactory we mean not

only consistent with existing experiment, but also incorporating other seemingly

secure parts of modern physics such as special relativity, field theory, and the stan-

dard model of elementary particle interactions. As Steve Weinberg summarized the

situation, “It is striking that it has not so far been possible to find a logically con-

sistent theory that is close to quantum mechanics other than quantum mechanics

itself” [18]. Alternatives to quantum theory meeting the above criteria would be of

great interest if only to guide experiment.

There are several directions under investigation today which aim at a theory

from which quantum mechanics would be emergent. Neither space nor the author’s
competence permit an extensive discussion of these ideas. But we can mention some
of the more important ones.3
Bohmian mechanics [20] in its most representative form is a deterministic but
3The references to these ideas are obviously not exhaustive, nor are they necessarily current.
Rather, they are to typical sources. For an encyclopedic survey of different interpretations and
alternatives to quantum mechanics, see [19].